Work Vehicle

ABSTRACT

Controlling means of a work vehicle includes, as speed change output setting data indicative of relationship between operational positions of the operational pedal and outputs from a stepless speed change device, first data for setting a deceleration completion position of the operational pedal where the output from the stepless speed change device is zero to provide a larger operational amount of the operational pedal from a stepping release position and second data for setting the deceleration completion position to provide a smaller operational amount of the operational pedal from the stepping release position.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/419,548, filed Mar. 14, 2012, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a work vehicle.

More particularly, the present invention relates to a vehicle speedcontrol arrangement for a work vehicle, comprising:

-   -   speed change operating means for speed-changing a stepless speed        change device to change an output from the stepless speed change        device;    -   a brake device for braking a wheel with a braking force        according to an operational position of an operational pedal        which is automatically returned to a stepping release position;        a pedal sensor for detecting an operational position of the        operational pedal; and    -   controlling means for controlling an operation of the speed        change operating means based on the output from the pedal        sensor, thereby to change the output from the stepless speed        change device to a speed according to the output from the pedal        sensor in a speed change range between a set speed preset by a        speed change operational tool and a zero speed.

Also, the present invention relates to a transmission switchingarrangement for a work vehicle comprising:

-   -   transmission switching means having a plurality of hydraulic        clutches for switching over a transmission state of a power        transmission line including speed changing means;    -   controlling means for controlling operations of the transmission        switching means and the speed changing means; and    -   commanding means for commanding to the controlling means a        switchover of the transmission state by the transmission        switching means.

2. Description of Related Art

[1] As a vehicle speed control arrangement of the above-described type,there is known one configured such that a vehicle body is braked andstopped in response to a movement of an operational pedal beyond aboundary position which is located between a speed change braking rangeand a stop braking range to reach the stop braking range; and that thevehicle body is started in response to a movement thereof beyond theboundary position back to the speed change braking range (see e.g.Japanese Unexamined Patent Application Publication No. 2010-111353 (JP2010-111353 A)).

With the arrangement described in JP 2010-111353 A, in order to maintainthe braked and stopped state of the vehicle body, it is required to keepthe operational pedal in the stop braking range with a large steppingforce. Therefore, if reduction occurs in the stepping force to theoperational pedal when e.g. another operation is effected under thebraked and stopped state, there is high possibility of the operationalpedal being returned to the speed change braking range beyond theboundary position. With this, there was a risk that the vehicle body maybe started inadvertently.

One object of the present invention is to prevent the possibility ofinadvertent start of the vehicle body due to reduction in the steppingforce to the operational pedal due to execution of another operationwhile the vehicle body is kept braked and stopped by a steppingoperation on the operational pedal.

[2] As a transmission switching arrangement of the above-described type,there is one known configured such that a forward-reverse switchingdevice is provided as the transmission switching means, and when thecontrolling means receives from the commanding means a forward-reverseswitching command by the forward-reverse switching device, thecontrolling means controls pressures of a pair of hydraulic clutchesthereby to effect a forward-reverse switchover (see e.g. JapaneseUnexamined Patent Application Publication No. 2009-074617 (JP2009-074617 A).

With the arrangement described in JP 2009-074617 A, during a high speedtraveling, when the operational state of the forward/reverse switchingdevice is switched over based on an output from the commanding means forcommanding a forward-reverse switchover by the forward-reverse switchingdevice, the amount of energy absorbed by the hydraulic clutch becomesexcessive, so that there is the risk of burnout damage of the hydraulicclutches.

Thus, another object of the present invention is to restrict occurrenceof burnout damage of the hydraulic clutches which may be caused by aswitchover of a transmission state by the transmission switching meansduring a high speed traveling.

SUMMARY OF THE INVENTION

The above object described in [1] is fulfilled according to one aspectof the present invention as under:—

A vehicle speed control arrangement for a work vehicle, comprising:

-   -   speed change operating means for speed-changing a stepless speed        change device to change an output from the stepless speed change        device;    -   a brake device for braking a wheel with a braking force        according to an operational position of an operational pedal        which is automatically returned to a stepping release position;        a pedal sensor for detecting an operational position of the        operational pedal; and    -   controlling means for controlling an operation of the speed        change operating means based on the output from the pedal        sensor, thereby to change the output from the stepless speed        change device to a speed according to the output from the pedal        sensor in a speed change range between a set speed preset by a        speed change operational tool and a zero speed;

wherein said controlling means includes, as speed change output settingdata indicative of relationship between operational positions of theoperational pedal and outputs from the stepless speed change device,first data for setting a deceleration completion position of theoperational pedal where the output from the stepless speed change deviceis zero to provide a larger operational amount of the operational pedalfrom said stepping release position, and second data for setting saiddeceleration completion position to provide a smaller operational amountof the operational pedal from said stepping release position; and

wherein upon detection, from the output from said pedal sensor duringexecution of a controlling operation based on said first data, that theoperational position of the operational pedal is at a position on theside of a stepping limit position beyond said deceleration completionposition, said controlling means is switched over from a state forexecuting the controlling operation based on said first data to a statefor executing the controlling operation based on said second data.

With the above-described arrangement, during execution of a controllingoperation based on the first data, if the operational pedal is operatedto an operational position on the side of the stepping limit positionbeyond the deceleration completion position of the first data thereby tobrake and stop the vehicle body, at the time of movement of theoperational pedal beyond the deceleration completion position of thefirst data, the controlling means is switched from the state forexecuting the controlling operation based on the first data to a statefor executing a controlling operation based on the second data. Withthis, the vehicle will not be started unless the operational pedal isreturned to the deceleration completion position of the second data pastthe deceleration completion position of the first data.

Therefore, if reduction occurs in the stepping force to the operationalpedal when another operation is effected under the braked and stoppedstate and the operational pedal is returned to a position between thedeceleration completion position of the first data and the decelerationcompletion position of the second data, it is possible to preventinadvertent start of the vehicle body.

Therefore, it is possible to prevent the possibility of inadvertentstart of the vehicle body due to reduction in the stepping force to theoperational pedal due to execution of another operation while thevehicle body is kept braked and stopped by a stepping operation on theoperational pedal.

Preferably, in the above-described arrangement, acceleration completionpositions of the operational pedal for changing the outputs from thestepless speed change device based on the first data and the second datato said set speed are set such that the acceleration completion positionof the second data provides a smaller stepping operational amount fromsaid stepping release position than the acceleration completion positionof the first data.

With the above-described arrangement, with the first data, the outputfrom the stepless speed change device is changed according to a steppingoperation of the operational peal between the acceleration completionposition (deceleration start position) to the deceleration completionposition of the first data. Whereas, with the second data, the outputfrom the stepless speed change device is changed between theacceleration completion position (deceleration start position) of thesecond data which is set to provide a smaller stepping operationalamount from the stepping release position than the accelerationcompletion position of the first data, and the deceleration completionposition of the second data which is set to provide a smaller steppingoperational amount from the stepping release position than thedeceleration completion position of the first data.

With the above-described arrangement, the change amount of the outputfrom the stepless speed change device relative to the change amount ofthe operational position of the operational pedal based on the firstdata can be made similar or equal to the change amount of the outputfrom the stepless speed change device relative to the change amount ofthe operational position of the operational pedal based on the seconddata.

Accordingly, the operational feel of the speed change operation by theoperational pedal based on the first data can be made similar or equalto the operational feel of the speed change operation by the operationalpedal based on the second data. As a result, the operability of speedchange operations by the operational pedal can be improved.

Preferably, in the above-described arrangement, an operational range ofthe operational pedal, ranging from an operational position of theoperational pedal located between said deceleration completion positionof the first data and said deceleration completion position of thesecond data, to said stepping limit position, is set to a brakingoperational rage for operating said brake device.

With the above-described arrangement, with a speed change operation bythe operational pedal based on the first data, the output from thestepless speed change device is changed according to a stepping positionof the operational pedal in the speed change operational range. And,once the pedal reaches the braking operational range, the brake deviceis braked with speed reduction at the stepping operational position ofthe operational pedal. Therefore, the operability of speed changeoperations by the operational pedal can be improved and also thetraveling stop operation of the vehicle body can be effected in areliable manner with alleviation of the load applied to the brakedevice.

With the above-described arrangement, with a speed change operation bythe operational pedal based on the second data, the output from thestepless speed change device is changed according to a stepping positionof the operational pedal and the brake device is not braked in the speedchange operational range. Hence, the operability of speed changeoperations by the operational pedal can be improved in the entirety ofthe speed change operational range of the operational pedal.

The above object described in [2] is fulfilled according to one aspectof the present invention as under:—

A transmission switching arrangement for a work vehicle comprising:

-   -   transmission switching means having a plurality of hydraulic        clutches for switching over a transmission state of a power        transmission line including speed changing means;    -   controlling means for controlling operations of the transmission        switching means and the speed changing means;    -   commanding means for commanding, to the controlling means,        switchover of the transmission state by the transmission        switching means: and    -   detecting means for detecting a rotational speed of the power        transmission line;

wherein, upon receipt of a command from said commanding means foreffecting the switchover of the transmission state by the transmissionswitching means, said controlling means compares an output from saiddetecting means with a set speed for transmission state switchover whichis set in advance; and when the output from said detecting means isgreater than said set speed, said controlling means causes reduction inthe rotational speed of the power transmission line by a speed reducingoperation of the speed changing means and then effects the transmissionstate switchover by controlling pressure of the plurality of hydraulicclutches.

With the above-described arrangement, it is possible to reduce theamount of energy absorbed by the hydraulic clutches in the course oftransmission switchover by the transmission switching means. As aresult, occurrence of burnout damage to the hydraulic clutches can berestricted.

Further, in comparison with e.g. the arrangement of reducing arotational speed of the power transmission line by retaining thetransmission switching means to the neutral position, the rotationalspeed of the power transmission line can be reduced more largely andspeedily. As a result, the occurrence of burnout damage to the hydraulicclutches can be restricted even more effectively without deteriorationin the operability of the transmission state switching operation.

Preferably, in the above-described arrangement, said speed changingmeans comprises a mechanical stepless speed change device.

With the above-described arrangement, in comparison with e.g. anarrangement wherein the rotational speed of the power transmission lineis reduced by a brake device as an example of the speed changing means,the rotational speed of the power transmission line can be reduced moresmoothly, whereby the operability of the transmission state switchingoperation can be improved.

Preferably, in the above-described arrangement, said controlling meansis configured to reduce the rotational speed of the power transmissionline to the set speed for the transmission state switchover by the speedreducing operation of the speed changing means; and then to effect thetransmission state switchover by controlling pressure of the pluralityof hydraulic clutches.

With the above-described arrangement, it is possible to reduce theamount of energy absorbed by the hydraulic clutches in the course oftransmission state switchover by the transmission switching means tosuch a level as hardly causes any trouble in the hydraulic clutches. Asa result, burnout damage of the hydraulic clutches can be prevented.

Preferably, in the above-described arrangement, said controlling meansis configured to reduce the rotational speed of the power transmissionline by effecting the speed reducing operation of the speed changingmeans for a set period; and then to effect the transmission stateswitchover by controlling pressure of the plurality of hydraulicclutches.

With the above-described arrangement, it is possible to preventdeterioration in the operability of the transmission state switchingoperation as reduction in the rotational speed of the power transmissionline takes too long.

That is, it is possible to restrict occurrence of burnout damage to thehydraulic clutches while avoiding deterioration in the operability ofthe transmission state switching operation.

Preferably, in the above-described arrangement, said controlling meansis configured to start the transmission state switchover by controllingpressure of the plurality of hydraulic clutches; and then to cause, byan acceleration operation of the speed changing means, to increase therotational speed of the power transmission line to a speed prior to thespeed reducing operation by the speed changing means.

With the above-described arrangement, it is possible to automaticallyincrease the rotational speed of the power transmission line to a speedprior to deceleration by the speed changing means while effectingswitchover of the transmission state by the controlling operation of thecontrolling means.

That is, occurrence of burnout damage to the hydraulic clutches can berestricted with achieving simplification of the operation.

Other aspects of the work vehicle, as well as advantageous effects ofthe same, will be apparent by reading the detailed descriptionhereinafter with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall side view of a tractor,

FIG. 2 is an overall plan view of the tractor,

FIG. 3 is a side view in vertical section showing a transmissionarrangement,

FIG. 4 is a block diagram showing a controlling arrangement,

FIG. 5 is a view showing relationship between operational positions of abrake pedal and outputs from a stepless speed change device,

FIG. 6 is a side view showing operational positions of the brake pedal,

FIG. 7 is a timing chart showing relationship between outputs from thestepless speed change device and a forward clutch and a reverse clutch,and

FIG. 8 is a flowchart of a forward-reverse switchover control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention will be described next withreference to the accompanying drawings, in which a tractor isillustrated as an example of a work vehicle.

As shown in FIG. 1 and FIG. 2, in this tractor, a clutch housing 2 isconnected to a rear portion of an engine 1 mounted on a front portion ofthe tractor and to this clutch housing 2, there is connected atransmission case acting also as a frame (“a T/M case” hereinafter)which extends from the rear portion of the clutch housing 2 toward therear portion of the vehicle body. On right and left sides of the engine1, there are mounted a pair of right and left front wheels 4 which aresteerable and drivable. On the right and left sides of the rear portionof the T/M case 3, there are mounted a pair of right and left rearwheels 5 which are drivable and can be braked. Upwardly of the T/M case3, there are mounted a steering wheel 6 for steering the front wheels, adriver's seat 7, etc. together forming a boarding driving section 8.

As shown in FIG. 3, power from the engine 1 is transmitted to a mainspeed change device 11 via a main clutch 9 and a gear type powerdistribution mechanism 10. And, a speed-changed power from the mainspeed change device 11 is transmitted as a traveling power to asynchromesh type auxiliary speed change device 13 via a forward-reverseswitching device 12. Further, the speed-changed power from the auxiliaryspeed change device 13 is transmitted as a front wheel driving power toan electro hydraulic control type front wheel speed change device 14 anda front wheel differential device 15, etc and is transmitted also as arear wheel driving power to the right and left rear wheels 5 via a rearwheel differential 16, etc. Further, power from the engine 1 past themain speed change device 11 (non speed changed power) is transmitted asworking power to a power takeoff (PTO) shaft 19 via an electronichydraulic control type PTO clutch 17, a synchromesh type PTO speedchange device 18, etc.

As shown in FIG. 1 and FIG. 2, rearwardly of the T/M case 3, there aremounted a pair of right and left lift arms 20 which enable liftingup/down operations of a work implement (not shown) such as a rotarytiller or plow to be connected to the rear portion of this tractor; apair of right and left lift cylinders 21 for pivotally driving thecorresponding lift arms 20, and so on. The right lift arm 20 is coupledvia a rolling cylinder 23 to a right lower link 22 which is verticallypivotally coupled to a right rear lower portion of the T/M case 3 forimplement connection. The left lift arm 20 is coupled via a coupling rod24 to a left lower link 22 which is vertically pivotally coupled to aleft rear lower portion of the T/M case 3 for implement connection. Asthe right and left cylinders 21, single action type hydraulic cylindersare employed and as the rolling cylinder 23, a double-action typehydraulic cylinder is employed.

Namely, this tractor is capable of lifting up/down a work implementcoupled to its rear portion by operations of the right and left liftcylinders 21 and capable also of a rolling action by an operation of therolling cylinder 23. Further, in case a driven type work implement suchas a rotary tiller or the like is connected to its rear portion, thetractor is capable of driving this work implement by the working powertaken off the PTO shaft 19.

As shown in FIG. 4, the engine 1 is steplessly adjustable in its outputrotational speed between an idling rotational speed and a ratedrotational speed by an operation of an acceleration cylinder 26 whichoperates a speed controlling lever 25 of its speed controller (notshown). As the acceleration cylinder 26, an electrically driven cylinderis employed. The operation of the acceleration cylinder 26 is controlledby acceleration controlling means 27A provided as a control program inan electronic control unit (“ECU” hereinafter) 27 which functions ascontrolling means.

The ECU 27 is constructed with using a microcomputer having a centralprocessing unit (CPU), an electrically erasable programmable read-onlymemory (EEPROM), etc. The ECU 27 inputs an output from a lever sensor 29for detecting an operational position of an accelerator lever 28, anoutput from an accelerator pedal 31 for detecting an operationalposition of an accelerator pedal 30, an output from an engine sensor 32for detecting an output rotational speed of the engine 1, etc. Further,the ECU 27 stores therein a first engine rotational speed setting dataindicative of relationship between operational positions of theaccelerator lever 28 and the output rotational speeds of the engine 1,and a second engine rotational speed setting data indicative ofrelationship between operational positions of the accelerator pedal 30and the output rotational speeds of the engine 1, etc.

As shown in FIG. 1, FIG. 2 and FIG. 4, the accelerator lever 28 isconstructed as a fore-aft pivotal position retaining type and is mountedon the right lower side of the steering wheel 6. The accelerator pedal30 is constructed as a self returning type which automatically returnsto a stepping release position and is mounted at the right foot rootportion of the boarding driving section 8. As the lever sensor 29 forthe accelerator lever and the pedal sensor 31 for the accelerator pedal,rotary type potentiometers are employed. As the first engine rotationalspeed setting data and the second engine rotational speed setting data,map data, relational expressions or the like can be employed.

The acceleration controlling means 27A effects accelerator control,based on the output from the lever sensor 29 for the accelerator lever,the pedal sensor 31 for the accelerator pedal, the output from theengine sensor 32 and so on. More particularly, a set rotational speedcorresponding to an operational position of the accelerator lever 28 anda set rotational speed corresponding to an operational position of theaccelerator pedal 30 are obtained based on the output from the leversensor 29 for the accelerator lever, the output from the pedal sensor 31for the accelerator pedal, the first engine rotational speed settingdata and the second engine rotational speed setting data; and theseobtained rotational speed set by the accelerator lever 28 and rotationalspeed set by the accelerator pedal 30 are compared with each other. And,if the rotational speed set by the accelerator pedal 30 is smaller thanor equals to the rotational speed set by the accelerator lever 28, therotational speed set by the accelerator lever 28 is employed as thetarget rotational speed. Whereas, if the rotational speed set by theaccelerator pedal 30 is greater than the rotational speed set by theaccelerator lever 28, the rotational speed set by the accelerator pedal30 is employed as the target rotational speed. And, the operation of theaccelerator cylinder 26 is controlled such that the output from theengine sensor 32 may agree to the employed target rotational speed (mayconfine within the insensitive bandwidth of the target rotationalspeed).

As shown in FIG. 3, as the main speed change device 11, there isemployed a hydromechanical type stepless speed change device (“HMT”hereinafter) 33 as an example of the stepless speed change device A. TheHMT 33 comprises combination of a hydrostatic stepless speed changedevice (“HST” hereinafter) 34 and a planetary gear mechanism 35. The HST34 includes an axial plunger type variable displacement pump 34A, anaxial plunger type fixed displacement pump 34B, and so on. The planetarygear mechanism 35 includes a sun gear 35A mounted at the center thereof,three planetary gears 35B mounted in equidistantly spaced relationshiparound the sun gear 35A to mesh therewith, a planetary carrier 35Csupporting the respective planetary gears 35B relatively rotatable witheach other, an outer gear 35D surrounding these planetary gears 35B andso on. Power from the power distribution mechanism 10 is transmitted toa pump shaft 34Aa of the HST 34 and the planetary carrier 35C of theplanetary gear mechanism 35. The HST 34 transmits the power after speedchanging by this HST 34 from a motor shaft 34Ba to the sun gear 35A ofthe planetary gear mechanism 35. The planetary gear mechanism 35combines and synthesizes the power from the engine 1 transmitted to theplanetary carrier 35C and the power transmitted to the sun gear 35 afterthe speed changing by the HST 34 and transmits the resultant power to aforward-reverse switching device 12.

As shown in FIG. 3 and FIG. 4, the HST 34 is capable of speed-changingthe power from the engine 1 by changing a swash plate angle of thevariable displacement pump 34A. The swash plate angle of the variabledisplacement pump 34A can be changed in stepless manner by an operationof a speed change cylinder 36 mounted inside the T/M case 3. As thisspeed change cylinder 36, a double-action type hydraulic cylinder isemployed. The operation of the speed change cylinder 36 is controlled byan operation of a speed change valve 37 which controls flow of oil tothe speed change cylinder 36. As the speed change valve 37, anelectromagnetic proportional valve is employed. The operation of thespeed change valve 37 is controlled by a controlling operation ofvehicle speed controlling means 27B provided as a control program in theECU 27.

As shown in FIG. 2 and FIG. 4, the ECU 27 inputs the output from a leversensor 39 for detecting the operational position of a main speed changevalve lever 38 as a speed change operational tool, an output from aswash plate angle sensor 40 for detecting the swash plate angle of thevariable displacement pump 34A, and so on. The ECU 27 further storestherein swash plate angle setting data indicative of relationshipbetween operational positions of the main speed change valve lever 38and the swash plate angles of the variable displacement pump 34A, and soon.

The main speed change lever 38 is constructed as a fore-aft pivotalposition retaining type and is mounted on the front side of an armrest41 mounted on the right side of the driver's seat 7. As the lever sensor39 for the main speed change lever and the swash plate angle sensor 40,rotary type potentiometers are employed. As the swash plate anglesetting data, map data, relational expressions or the like can beemployed.

The vehicle speed controlling means 27B effects a main speed changecontrol, based on the output from the lever sensor 39 for the main speedchange lever, the output from the swash plate angle sensor 40, and theswash plate angle setting data. More particularly, a swash plate angleof the variable displacement pump 34A is set as the target swash plateangle, which angle corresponds to the operational position of the mainspeed change lever 38 which is obtained based on the operationalposition of the main speed change lever 38 outputted from the leversensor 39 for the main speed change lever and also based on the swashplate angle setting data. Then, the operation of the speed changecylinder 36 is controlled by controlling the speed change valve 37 suchthat the swash plate angle of the variable displacement pump 34A mayagree to this target swash plate angle (the swash plate angle of thevariable displacement pump 34A may be confined within the insensitivebandwidth of the target swash plate angle).

Though not shown, the swash plate angle setting data are configured forsetting the relationship between the operational positions of the mainspeed change lever 38 and the swash plate angles of the variabledisplacement pump 34A as follows. When the operational position of themain speed change lever 38 is at the zero speed position, the swashplate angle of the variable displacement pump 34A is set to the maximumangle in the reverse direction and the operational position of the pumpswash plate 34Ab is set to the reverse highest speed position. Further,when the operational position of the min speed change lever 38 is at thehighest speed position, the swash plate angle of the variabledisplacement pump 34 a is set to the maximum angle in the forwarddirection and the operational position of the pump swash plate 34Ab isset to the forward highest speed position. In these ways, theoperational amount of the pump swash plate 34Ab from the reverse highestspeed position is changed in accordance with the operational amount ofthe main speed change lever 38 from the zero speed position.

The HMT 33 is configured as follows. Namely, when the swash plate angleof the variable displacement pump 34A is the maximum angle in thereverse direction (in the following discussion, this will be referred toas the minimum swash plate angle of the variable displacement pump 34A),the output after the synthesis by the planetary gear mechanism 35becomes the zero speed. And, when the swash plate angle of the variabledisplacement pump 34A is the maximum angle in the forward direction (inthe following discussion, this will be referred to as the maximum swashplate angle of the variable displacement pump 34A), the output after thesynthesis by the planetary gear mechanism 35 becomes the highest speed.In these ways, the output is increased in accordance with theoperational amount from the minimum swash plate angle (the reversehighest speed position of the pump swash plate 34Ab) of the variabledisplacement pump 34A.

That is, the speed change cylinder 36, the speed change valve 37, and soon together constitute speed change operating means B for varying theoutput from the HST 33 with an operation of the pump swash plate 34Ab ofthe variable displacement pump 34A.

Incidentally, the swash plate angle setting data are set such that aproportional relationship may be established between the operationalpositions of the main speed change lever 38 and the outputs from the HMT33.

As shown in FIGS. 1 through 4, the forward-reverse switching device 12comprises a forward clutch 12A and a reverse clutch 12B employingmultiple-plate hydraulic clutches, etc. And, by controlling operationsof a forward-reverse switching valve 42 for controlling flow of oilto/from the forward clutch 12A and the reverse clutch 12B, there can beselectively provided a forward transmission state wherein the power fromthe HMT 33 is transmitted as a forward traveling power to the auxiliaryspeed change device 13, a reverse transmission state wherein the powerfrom the HMT 33 is transmitted as a reverse traveling power to theauxiliary speed change device 13, and a transmission blocked statewherein the power transmission from the HMT 33 to the auxiliary speedchange device 13 is blocked. As the forward-reverse switching valve 42,an electromagnetic control valve is employed. The operation of theforward-reverse switching valve 42 is controlled by a controllingoperation of forward-reverse switchover controlling means 27C providedas a control program in the ECU 27.

As shown in FIG. 2 and FIG. 4, the ECU 27 inputs e.g. an output from aforward/reverse (FR) sensor 44 for detecting an operational position ofan FR lever 43 for forward-reverse switchover. The FR lever 43 isprovided as a forward-reverse pivotal type capable of retaining theposition selectively at two positions of a forward position and areverse position and is mounted on the left lower side of the steeringwheel 6. The FR switch 44 includes a forward position detecting microswitch which is switched to its closed state in response to an operationof the FR lever 44 to the forward traveling position, and a reverseposition detecting micro switch which is switched to its closed state inresponse to an operation of the FR lever 44 to the reverse travelingposition.

The forward-reverse switchover controlling means 27C effectsforward-reverse switchover control for switching over the operationalstate of the forward-reverse switching device 12, based on the outputfrom the FR sensor 44. In this forward-reverse switchover control,basically, upon detection of switchover of the FR lever 43 from theforward position to the reverse position based on the output from the FRsensor 44, the forward clutch 12A is depressurized to be switched to theblocking state, and upon lapse of a set period thereafter, the reverseclutch 12B is pressurized to be switched to its transmission state, suchthat the forward-reverse switching device 12 is switched to the reversetransmission state by controlling the operation of the forward-reverseswitching valve 42. Upon detection of switchover of the FR lever 43 fromthe reverse position to the forward position based on the output fromthe FR sensor 44, the reverse clutch 12B is depressurized to be switchedto the blocking state, and upon lapse of a set period thereafter, theforward clutch 12A is pressurized to be switched to its transmissionstate, such that the forward-reverse switching device 12 switched to theforward transmission state by controlling the operation of theforward-reverse switching valve 42.

As shown in FIG. 2, in the boarding driving section 8 and on the leftside portion of the driver's seat 7, there is mounted an auxiliary speedchange lever 45. Though not shown, the auxiliary speed change device 13is operably coupled to this auxiliary speed change lever 45 via amechanical type auxiliary speed change coupling mechanism such that thespeed position of the auxiliary speed change device 13 is switched to alow speed for working, a high speed for working or to a highest speedfor traveling in accordance with the operational position of theauxiliary speed change lever 45.

As shown in FIG. 3, with a sliding operation of a shift member 46, thefront wheel speed change device 14 is capable of selectively providingan equal speed transmission state wherein the power from the auxiliaryspeed change device 13 is transmitted to the right and left front wheels4 such that the peripheral speed of the right and left front wheels 4may synchronize with the peripheral speed of the right and left rearwheels 5, or an accelerated transmission state wherein the power fromthe auxiliary speed change device 13 is transmitted to the right andleft front wheels 4 such that the peripheral speed of the right and leftfront wheels 4 may be approximately the double speed of the peripheralspeed of the right and left rear wheels 5.

Though not shown, the shift member 46 slides the front wheel speedchange device 14 to the equal speed transmission state or to theaccelerated transmission state, with an operation of the front wheelspeed change cylinder employing a double-action type hydraulic cylinder.The operation of this front wheel speed change cylinder can becontrolled by controlling the operation of the front wheel speed changevalve which controls the flow of oil to/from the front wheel speedchange cylinder. As the front wheel speed change valve, anelectromagnetic control valve is employed. The operation of the frontwheel speed change valve is controlled by a controlling operation offront wheel speed change controlling means provided as a program in theECU 27.

The ECU 27 inputs an output from a steering angle sensor detecting asteering angle of the front wheels 4, an output from a selector switchprovided in the boarding driving section 8, etc. As the steering anglesensor, a rotary type potentiometer is employed.

The front wheel speed change controlling means is configured to becapable of effecting a front wheel speed change control for switchingover the front wheel speed change device 14 between the equal speedtransmission state and the accelerated transmission state according tothe output from the steering angle sensor. And, in response to eachoutput from an ON signal from the selector switch with a pressingoperation of this selector switch, switchover is effected between afront wheel speed change selected state for effecting the front wheelspeed change control, and a front wheel speed control release state fornot effecting the front wheel speed change control.

In the front wheel speed change control, specifically, the operation ofthe front wheel speed change valve is controlled as follows. Namely,when it is detected that the steering angle of the front wheels 4 issmaller than a set angle (e.g. 35 degrees) based upon the output fromthe steering angle sensor, the front wheel speed change device 14 isrendered into the equal speed transmission state. Whereas, when it isdetected that the steering angle of the front wheels 4 is greater thanthe set angle (e.g. 35 degrees) based upon the output from the steeringangle sensor, the front wheel speed change device 14 is rendered intothe accelerated transmission state.

With the above-described arrangement, when it is desired to reduce thesteering radius of the vehicle body at the time of e.g. a furrow edgeturn for steering the front wheels 4 by an angle greater than the setangle, the state will be set in advance to the front wheel speed changeselected state by an operation of the selector switch. With this, inassociation with the steering of the front wheels 4 by the angle greaterthan the set angle, the front wheel speed change device 14 can beautomatically switched from the equal speed transmission state to theaccelerated transmission state, so that the vehicle can be turned with aturning radius which is smaller than the turning radius at the time ofequal speed transmission state. And, by returning the steering angle ofthe front wheels 4 to an angle smaller than the set angle, the frontwheel speed change device 14 can be automatically switched from theaccelerated transmission state back to the equal speed transmissionstate.

As shown in FIG. 1, FIG. 2 and FIG. 4, at the right foot base portion ofthe boarding driving section 8, there are mounted a pair of right andleft brake pedals 47 as operational pedals C. Each of the right and leftbrake pedals 47 is pivotally returned to a stepping release position bythe action of a torsion spring 48 corresponding thereto. The left brakepedal 47 is operably coupled via a left mechanical brake couplingmechanism 50 to a left side brake 49 which brakes the left rear wheel 5.The right brake pedal 47 is operably coupled via a right mechanicalbrake coupling mechanism 50 to a right side brake 49 which brakes theright rear wheel 5. As the right and left side brakes, multiple-platetype brakes are employed.

Though not shown, the left brake coupling mechanism 50 operably couplesthe left brake pedal 47 and the left side brake 49 to each other via acrank arm, a push rod, etc, so that when the left brake pedal 47 isstepped on to a braking operational range, the left side brake 49 iscaused to brake the left rear wheel 5 by a braking force correspondingto the amount of the stepping operation of the left brake pedal 47 inthe braking operational range. Similarly, the right brake couplingmechanism 50 operably couples the right brake pedal 47 and the rightside brake 49 to each other via a crank arm, a push rod, etc, so thatwhen the right brake pedal 47 is stepped on to a braking operationalrange, the right side brake 49 is caused to brake the right rear wheel 5by a braking force corresponding to the amount of the stepping operationof the right brake pedal 47 in the braking operational range.

With the above-described arrangement, by stepping on the left brakepedal 47 alone to the braking operational range, the left rear wheel 5can be braked by the left side brake 49. Conversely, by stepping on theright brake pedal 47 alone to the braking operational range, the rightrear wheel 5 can be braked by the right side brake 49. Further, bystepping on the right and left brake pedals 47 at the same time by asame operational amount to the respective braking operational ranges,the right and left rear wheels 5 can be braked at the same time with asame braking force by the right and left side brakes 49.

That is to say, at the time of a turning traveling with a pivotingoperation of the steering wheel 6 to the steering direction, by steppingeither one of the right and left brake pedals 47 alone which one pedalcorresponds to the inner turning side rear wheel 5, the turning statecan be switched from the turning state provided by the pivotal operationof the steering wheel 6 to the braking turning state for braking theturning inner side rear wheel, so that the turning radius of the vehiclebody can be reduced. Further, by stepping on the right and left brakepedals 47 at the same time to the braking operational range, the rightand left side brakes 49 can be used as a deceleration stopping brakedevice D.

Though not shown, the right and left brake pedals 47 are provided with acoupling mechanism capable of selectively providing a coupled state forinhibiting their independent operations and a coupling released statefor allowing the independent operations. With this, the right and leftbrake pedals 47 can be switched between a state which allows switchoverto the braking turning state by their independent operations and a statefor inhibiting switchover to the braking turning state.

Further, the boarding driving section 8 is provided with a retainingmechanism capable of retaining the left brake pedal 47 stepped on to thebraking operational range to this braking operational range. With this,after the right and left brake pedals 47 are stepped on to the brakingoperational range while these pedals are coupled to each other via thecoupling mechanism, the retaining mechanism will be operated to retainthe left brake pedal 47 to the braking operational range, whereby theright and left brake pedals 47 can be retained at the respective brakingoperational range at that time. As a result, the right and left sidebrakes 49 can be retained at the braking state wherein the right andleft rear wheels 5 are braked by a braking force corresponding to theamount of the stepping operation on the right and left brake pedals 47.That is, by the retaining mechanism, the right and left side brakes 49can be caused to function as a parking brake.

As shown in FIG. 4, to the right and left brake pedals 47, there isoperably coupled a single pedal sensor 51 for detecting a simultaneousstepping position of the right and left brake pedals 47 which is theposition of the operation of stepping on these brake pedals 47 at thesame time, via a link mechanism 52 configured to transmit, to the pedalsensor 51, only the amount of simultaneous stepping operation from thestepping release position in case the right and left brake pedals 47 arestepped on at the same time.

As the pedal sensor 51, a rotary potentiometer is employed. And, thispedal sensor 51 detects the amount of the simultaneous steppingoperation from the stepping release position of the right and left brakepedals 47, as the simultaneous stepping operational position of theright and left brake pedals 47.

The link mechanism 52 is configured to maintain, by the action of atorsion spring (not shown), a right-left oriented contact member 52Adisposed upstream in the stepping direction of the right and left brakepedals 47 in contact with an arm portion 47A of either one brake pedal47 from the upstream side in the stepping direction. With thisarrangement, when the right and left brake pedals 47 are not stepped onsimultaneously, the contact member 52A is received by the right and leftbrake pedals 47 positioned at the stepping release position, thus beingpositioned at its reference position which corresponds to the steppingrelease position. Whereas, when either one of the right and left brakepedals 47 is stepped on, the contact member 52A is received by the otherbrake pedal 47 which is positioned at the stepping release position,thus being positioned at the reference position. Only when the right andleft brake pedals 47 are stepped on simultaneously, the contact member52A follows to come into contact with the right and left brake pedals 47with the action of the torsion spring. And, the following movementamount of the contact portion 52A from the reference position istransmitted to the pedal sensor 51 as a simultaneous steppingoperational amount from the stepping release position of the right andleft brake pedals 47.

As shown in FIG. 4 and FIG. 5, the pedal sensor 51 outputs the detectedsimultaneous stepping operational position of the right and left brakepedals 47 to the ECU 27. The ECU 27 stores therein e.g. speed changeoutput setting data indicative of relationship between the simultaneousstepping operational positions of the right and left brake pedals 47 andthe outputs from the HMT 33. The speed change output setting data can bemap data, relational expressions, or the like. Incidentally, this speedchange output setting data serves for both forward traveling and reversetraveling. Instead, speed change output setting data for forwardtraveling and speed change output setting data for reverse traveling maybe provided separately.

In the ECU 27, the vehicle speed controlling means 27B effects a pedalspeed change control, based on a set swash plate angle of the variabledisplacement pump 34A set in correspondence with the operationalposition of the main speed change lever 38, the output from the swashplate angle sensor 40, the output from the pedal sensor 51 for the brakepedal and the speed change output setting data. More particularly, a setoutput (set speed) of the HMT 33 corresponding to the operationalposition of the main speed change lever 38 is obtained from a set swashplate angle of the variable displacement pump 34A set in correspondencewith the operational position of the main speed change lever 38. Then,the speed change output setting data are corrected such that this setoutput may become the maximum output (maximum speed) corresponding tothe stepping release position (a) of the right and left brake pedals 47based on the speed change output setting data. And, based on thesimultaneous stepping position of the right and left brake pedals 47outputted from the pedal sensor 51 for the brake pedal and on thecorrected speed change output setting data, the output from the HMT 33(output speed) corresponding to the simultaneous stepping position ofthe right and left brake pedals 47 is obtained and the obtained outputfrom the HMT 33 is set as a target output (target speed). Then, a targetswash plate angle for the variable displacement pump 34A correspondingto a target output for obtaining this target output as the output fromthe HMT 33 is obtained and the speed change cylinder 36 is operated bycontrolling the operation of the speed change valve 37 so that the swashplate angle of the variable displacement pump 34A may agree to theobtained target swash plate angle (may confine within the insensitiveband with of the target swash plate angle).

As shown in FIG. 5 and FIG. 6, the speed change output setting dataincludes first data and second data. The first data sets relationshipbetween the simultaneous stepping operational positions of the right andleft brake pedals 47 and the outputs from the HMT 33 as follows. Namely,during movement of the right and left brake pedals 47 from the steppingrelease position (a) to a preset deceleration start position(acceleration complete position) (b), the output from the HMT 33 ismaintained at the maximum output (the set speed corresponding to anoperational position of the main speed change valve lever 38). Duringthe movement from the arrival at the deceleration start position (b) andthen beyond a deceleration completion position (c), the output from theHMT 33 is changed between the maximum output and the minimum output(zero speed) in reverse proportion to the simultaneous steppingoperational amount of the right and left brake pedals 47 (see the solidline in FIG. 5). The second data sets relationship between thesimultaneous stepping operational positions of the right and left brakepedals 47 and the outputs from the HMT 33 as follows. Namely, duringmovement of the right and left brake pedals 47 from a stepping limitposition (d) to a preset acceleration start position (decelerationcompletion position) (e), the output from the HMT 33 is maintained tothe minimum speed. The, during the movement from the acceleration startposition (e) back before a present acceleration completion position (f),the output from the HMT 33 is changed between the minimum output and themaximum output in reverse proportion to the simultaneous steppingoperational amount of the right and left brake pedals 47 (see the brokenline in FIG. 5).

The deceleration start position (b) is set at a operational position ofa larger simultaneous stepping operational amount from the steppingrelease position of the right and left brake pedals 47 than theacceleration completion position (f). The acceleration start position(e) is set at an operational position of a smaller simultaneous steppingoperational amount from the stepping release position (a) of the rightand left brake pedals 47 than the deceleration completion position (c).

Further, the difference between one simultaneous stepping operationalamount of the right and left brake pedals 47 from the stepping releaseposition (a) to the deceleration start position (b) and the othersimultaneous stepping operational amount from the stepping releaseposition (a) to the acceleration completion position (f) is set smallerthan the difference between one simultaneous stepping operational amountof the right and left brake pedals 47 from the stepping release position(a) to the deceleration completion position (c) and the othersimultaneous stepping operational amount from the stepping releaseposition (a) to the acceleration start position (e). Whereby, the changeamount of the output from the HMT 33 corresponding to the simultaneousstepping operational amount of the right and left brake pedals 47between the deceleration start position (b) and the decelerationcompletion position (c) of the first data may be smaller than the changeamount of the output from the HMT 33 corresponding to the simultaneousstepping operational amount of the right and left brake pedals 47between the acceleration start position (e) and the accelerationcompletion position (f) of the second data:

The vehicle speed controlling means 27B is configured to switch over thespeed change output setting data from the first data to the second datafor effecting a pedal speed change control based on the second data,when the simultaneous stepping operational position of the right andleft brake pedals 47 moves beyond the deceleration completion position(c) during a pedal speed change control based on the first data.Further, the vehicle speed controlling means 27B is configured also toswitch over the speed change output setting data from the second data tothe first data for effecting a pedal speed change control based on thefirst data, when the simultaneous stepping operational position of theright and left brake pedals 47 moves back before the accelerationcompletion position (f) during a pedal speed change control based on thesecond data.

The operational range of the right and left brake pedals 47 includes abraking release range wherein the right and left side brakes 49 aremaintained to the braking release state, despite a stepping operation ofthe right and left brake pedals 47; and a braking operational rangewherein, the braking force applied by the right/left side brake 49 tothe corresponding rear wheel 5 is increased in response to increase inthe stepping operational amount of the right and left brake pedals 47,and the braking force applied by the right/left side brake 49 to thecorresponding rear wheel 5 is decreased in response to decrease in thestepping operational amount of the right and left brake pedals 47. And,a boundary position (g) between the braking release range and thebraking operational range is located between the deceleration completionposition (c) and the acceleration start position (e), so that thedeceleration completion position side range portion in the speed changeoperational range of the right and left brake pedals 47 of the firstdata is overlapped with the braking operational range and that the speedchange operational range of the right and left brake pedals 47 of thesecond data is not overlapped with the braking operational range.

With the above-described arrangement, during a traveling condition wherethe main speed change valve lever 38 is operated to a desiredoperational position and the right and left brake pedals 47 arepositioned at the stepping release position (a), if a simultaneousstepping operation of the right and left brake pedals 47 is effected,the vehicle speed controlling means 27B effects the pedal speed changecontrol based on the first data, and the right and left side brakes 49brake the right and left rear wheels 5 with a braking forcecorresponding to the simultaneous stepping operational amount in thebraking operational range.

With the above, during the movement of the right and left brake pedals47 from the stepping release position (a) to the deceleration startposition (b), the vehicle speed controlling means 27B maintains theoutput from the HMT 33 to a set speed corresponding to the operationalposition of the main speed change valve lever 38 and also the right andleft side brakes 49 do not brake the right and left rear wheels 5.Hence, despite a simultaneous stepping operation of the right and leftbrake pedals 47, the vehicle speed change can be fixed at the set speedobtained by the operational positions of the accelerator lever 28, themain speed change valve lever 38 and the auxiliary speed change valvelever 45.

Further, during the movement of the right and left brake pedals 47 fromthe deceleration start position (b) past the boundary position (g)between the braking release position and the braking operationalposition, the vehicle speed controlling means 27B changes the outputfrom the HMT 33 according to the simultaneous stepping operationalposition of the right and left brake pedals 47 and the right and leftside brakes 49 do not brake the rear wheels 5. Therefore, with asimultaneous stepping operation of the right and left brake pedals 47,the vehicle speed can be changed within a range between the set positionand a speed obtained when the right and left brake pedals 47 are steppedon to the boundary position (g).

Moreover, during the further movement of the right and left brake pedals47 from the boundary position (g) and past the deceleration completionposition (c), the vehicle speed controlling means 27B changes the outputfrom the HMT 33 according to the operational position of the right andleft brake pedals 47 and also the right and left side brakes 49 brakethe right and left rear wheels 5 with a braking force corresponding tothe simultaneous stepping operational amount in the braking operationalrange of the right and left brake pedals 47. Hence, the vehicle speedcan be changed between a speed obtained when the right and left brakepedals 47 are stepped on to the boundary position (g) and the zerospeed; and also, the vehicle body can be braked and stopped at the timeof or prior to arrival of the right and left bake pedals 47 at thedeceleration completion position (c).

And, when the right and left brake pedals 47 move beyond thedeceleration completion position (c), the vehicle speed controllingmeans 27B is switched over from a state for executing the pedal speedchange control based on the first data to a state for executing thepedal speed change control based on the second data. Whereby, while theright and left brake pedals 47 move from the stepping limit position (d)beyond the deceleration completion position (c) to arrive at theboundary position (g), the vehicle speed controlling means 27B maintainsthe output from the HMT 33 at the zero speed and the right and left sidebrakes 49 brake the rear wheels 5 with a braking force corresponding tothe simultaneous stepping operational amount in the braking operationalrange of the right and left brake pedals 47. As a result, the vehiclebody can be maintained under the braked and stopped state even if thestepping force onto the right and left brake pedals 47 is reduced tobring the right and left brake pedals 47 closer to the boundary position(g) than the deceleration completion position (c).

Further, during the movement of the right and left brake pedals 47 fromthe boundary position (g) back to the acceleration start position (e),the vehicle speed controlling means 27B maintains the output from theHMT 33 to zero speed, thus constantly maintaining the vehicle speed tozero. As a result, the vehicle body can be maintained under the brakedand stopped state, even if the stepping force onto the right and leftbrake pedals 47 is reduced to bring the right and left brake pedals 47closer to the acceleration start position (e) than to the boundaryposition (g) and release the braking of the right and left rear wheels 5by the right and left side brakes 49.

Furthermore, during the movement of the right and left brake pedals 47from the acceleration start position (e) back to the accelerationcompletion position (f), the vehicle speed controlling means 27B changesthe output from the HMT 33 according to the simultaneous steppingoperational position of the right and left brake pedals 47 and the rightand left side brakes 49 do not brake the right and left rear wheels 5.Hence, the vehicle speed can be changed between the zero speed and a setspeed with a simultaneous stepping operation of the right and left brakepedals 47.

And, when the right and left brake pedals 47 move past the accelerationcompletion position (f), the vehicle speed controlling means 27B isswitched over from the state for executing the pedal speed changecontrol based on the second data to the state for executing the pedalspeed change control based on the first data. And, as describedhereinbefore, during the movement of the right and left brake pedals 47from the stepping release position (a) to the deceleration startposition (b), the vehicle speed can be maintained fixed at the setspeed, despite a simultaneous stepping operation of the right and leftbrake pedals 47.

That is, the right and left brake pedals 47 can be used as speed changeoperational pedals for changing the vehicle speed between the zero speedand a set speed. With this, speed change operations can be made simple.

Further, under the condition where the vehicle body is braked andstopped as a result of a simultaneous stepping operation of the rightand left brake pedals 47 to an operational position on the steppinglimit position side beyond the deceleration completion position (c), thevehicle body will not be started unless the right and left brake pedals47 are moved beyond the deceleration completion position (c) back to theacceleration start position (e). Therefore, it is possible to preventinadvertent start of the vehicle body even when the right and left brakepedals 47 are returned to a position between the deceleration completionposition (c) and the acceleration start position (e) due to reduction inthe stepping force to the right and left brake pedals 47 as a result ofe.g. execution of another operation while the vehicle body is braked andstopped with the simultaneous stepping operation of the right and leftbrake pedals 47.

Moreover, during the execution of the pedal speed change control basedon the first data by the vehicle speed controlling means 27B, thereoccurs no switchover to the pedal speed change control based on thesecond data unless the right and left brake pedals 47 move beyond thedeceleration completion position (c). Therefore, during a simultaneousstepping operation of the right and left brake pedals 47 between thedeceleration start position (b) and the deceleration completion position(c), there is provided a same operational feel without a change in therelationship between the simultaneous stepping operational positions ofthe right and left brake pedals 47 and the vehicle speeds. Also, duringexecution of the pedal speed change control based on the second data bythe vehicle speed controlling means 27B, no switchover to the pedalspeed change control based on the first data occurs, unless the rightand left brake pedals 47 move beyond the acceleration completionposition (f). Therefore, during the simultaneous stepping operation ofthe right and left brake pedals 47 between the deceleration startposition (b) and the deceleration completion position (c), no changeoccurs in the relationship between the simultaneous stepping operationalpositions of the right and left brake pedals 47 and the vehicle speeds.Hence, the operational feel is kept same. Consequently, this willfacilitate e.g. an inching operation for effecting start and stop of thevehicle body in repetition.

As shown in FIG. 4, FIG. 7 and FIG. 8, during a high speed traveling,when the operational state of the forward-reverse switching device 12acting as transmission switching means E is switched based on an outputfrom the FR sensor 44 acting as commanding means F for commandingforward-reverse switchover by the forward-reverse switching device 12,this results in increase in the amount of energy absorbed by the forwardclutch 12A and the reverse clutch 12B, so that there is the risk ofburnout damage to the forward clutch 12A and the reverse clutch 12B. Forthis reason, the forward-reverse switchover controlling means 27C isconfigured to effect a controlling operation as follows for avoidingoccurrence of such burnout.

First, the process determines whether or not a switchover operation ofthe FR lever 44 to the forward traveling position or the reversetraveling operation has been detected based on the output from the FRsensor 44 (step #1). If no switchover operation is detected, the processreturns to step #1. Whereas, if a switchover operation is detected,based on the output from the engine sensor 32 and the output from theswash plate angle sensor 40, the process calculates the output speed ofthe HMT 33 (the input speed for the forward-reverse switching device 12)(step #2) and then determines whether or not the output speed of the HMT33 exceeds a set speed (e.g. 15 Km/h) for the forward-reverse switchoverwhich is preset for burnout damage prevention (step #3).

If the set speed for burnout damage prevention is not exceeded, theforward clutch 12A or the reverse clutch 12B whichever does notcorrespond to the operational position of the FR lever 43 isdepressurized to be switched to the blocking state. Then, upon lapse ofa set period thereafter, the forward clutch 12A or the reverse clutch12B whichever corresponds to the operational position of the FR lever 43is pressurized to be switched to the transmission state. In this way,clutch pressure switching control for controlling operations of theforward-reverse switch valve 42 is effected (step #4). Upon completionof the clutch pressure switchover control, the forward-reverseswitchover control is ended.

In case the set speed for burnout damage prevention is exceeded, adeceleration control is initiated so that the output speed of the HMT 33may be reduced with reduction in the swash plate angle of the variabledisplacement pump 34A with an operation of the speed change cylinder 36(step #5). Also, the process calculates the output speed of the HMT 33based on the output from the engine sensor 32 and the output from theswash plate angle sensor 40 (step #6) and measures the period from theinitiation of the deceleration control by means of a timer (not shown)included in the ECU 27.

Next, the process determines whether the output speed of the HMT 33 hasreached the set speed for burnout damage prevention or not (step #8). Ifthe set speed for burnout damage prevention has been reached, thedeceleration control is terminated and the output speed of the HMT 33 ismaintained to the set speed for burnout damage prevention (step #9).Also, the clutch pressure switching control is initiated (step #10) andthe process measures the period from the initiation of the clutchpressure switching control with the timer (step #11).

On the other hand, if the set speed for burnout damage prevention hasnot yet been reached at step #8, the process determines whether themeasured period from the initiation of the deceleration control haspassed a first set period or not (step #12). If the period has not yetbeen passed, the process returns to step #8. Whereas, if the period hasbeen passed, the process goes to step #9 to maintain the output speed ofthe HMT 33 to the current speed thereof.

After initiation of the clutch pressure switching control, the processdetermines whether the clutch pressure of the pressurizing side forwardclutch 12A or reverse clutch 12 b has reached a set pressure or not(step #13). If the pressurizing side clutch pressure has reached the setpressure, an acceleration control is effected for controlling theoperation of the speed change valve 37 such that the output speed of theHMT 33 may increase to the set speed corresponding to the operationalposition of the main speed change valve lever 38 with increase of theswash plate angle of the variable displacement pump 34A (step #14).

On the other hand, if the pressurizing side clutch pressure has not yetreached the set pressure, the process determines whether the measuredperiod from the initiation of the pressure switching control has passeda second period or not (step #15). If the period has not yet beenpassed, the process returns to step #13. Whereas, if the period has beenpassed, the process goes to step #14 to effect the acceleration control.

Then, the process determines whether the clutch pressure switchingcontrol has been completed or not (step #16). Upon completions of theclutch pressure switching control and the acceleration control, theforward-reverse switchover control is terminated.

That is, in case the output speed of the HMT 33 during the forward andreverse switchover operation based on the operation of the FR lever 43is a speed which can be reduced to the set speed for burnout damageprevention within the first set period with the deceleration control,burnout damage to the forward clutch 12A and the reverse clutch 12B canbe avoided without giving any unnatural feel to the driver withprolongation of the forward-reverse switching period. Further, even ifthe output speed of the HMT 33 is not a speed which can be reduced tothe set speed for burnout damage prevention within the first period,burnout damage to the forward clutch 12A and the reverse clutch 12B canbe avoided also without giving any unnatural feel to the driver withprolongation of the forward-reverse switching period.

Incidentally, with this tractor, the engine sensor 32, the swash plateangle sensor 40 and so on together constitute detecting means G fordetecting the output speed of the HMT 33 as the rotational speed of thepower transmission line including the forward-reverse switching device12. Further, the HMT 33 functions as speed changing means H for changingthe rotational speed of the power transmission line including theforward-reverse switching device 12 at the time of forward-reverseswitchover by an operation of this forward-reverse switching device 12.

Further Embodiments

-   -   [1] As the stepless speed change device A, a hydrostatic        stepless speed change device or a belt type stepless speed        change device, etc. can be employed.    -   [2] As the speed change operating means B, an electrically        driven cylinder etc. can be employed.    -   [3] As the operational pedal C, this can be a single brake        pedal. And, the brake device D can be a single brake device.        Further, this can be configured for braking the front wheels 4        or the rear wheels 5 or braking both the front wheels 4 and the        rear wheels 5.    -   [4] The pedal sensor 51 can be provided in correspondence with        each one of the right and left brake pedals 47.    -   [5] As the speed change operational tool 38, a switch or the        like can also be employed.    -   [6] The first data and the second data can be set such that        their acceleration completion positions (b), (f) are same.    -   [7] The operational range of the operational pedal C from the        deceleration completion position (e) to the stepping limit        position (d) of the second data can be set to the braking        operational range for operating the brake device D.    -   [8] The transmission switching means E can be not only a        hydraulic clutch type speed change device having a plurality of        hydraulic clutches 12A, 12B for effecting a multiple stages of        speed change operations, but also a front wheel speed change        device including a plurality of hydraulic clutches 12A, 12B for        changing the drive speed of the front wheels 4 in multiple        stages, etc. The number of the hydraulic clutches 12A, 12B can        be three or more.    -   [9] The commanding means F can be a two-position switchover        switch, an ON/OFF switch or the like.    -   [10] The detecting means G can be an engine sensor 32, an        electromagnetic pickup type rotational sensor for detecting an        output from the stepless speed change device A or can be an        electromagnetic pickup type rotary sensor for detecting a        vehicle speed.    -   [11] The speed changing means H can be the engine 1 and the        brake device D. Further, it can be effected by combinational        operations of the stepless speed change device A, the engine 1,        the brake device D, etc. Further, it is possible to configure        such that in case it is not possible to reduce the rotational        speed in the power transmission line to the set speed for        burnout damage prevention within the first set period with an        operation of a single device alone such as the stepless speed        change device A, the engine 1, or the brake device D, etc., the        reduction is effected by combination of the stepless speed        change device A, the engine 1, or the brake device D, etc.    -   [12] The controlling means 27 may be configured such that        switchover of the transmission state is effected by controlling        the pressures of the plurality of hydraulic clutches 12A, 12B        only when the rotational speed in the power transmission line is        reduced to the set speed by a deceleration operation of the        speed changing means H.    -   [13] The controlling means 27 may be configured such that        switchover of the transmission state is effected by controlling        the pressures of the plurality of hydraulic clutches 12A, 12B        only when the rotational speed in the power transmission line is        reduced by a deceleration operation of the speed changing means        H effected for a set period.    -   [14] A tractor is illustrated as one example of a work vehicle        in the foregoing embodiment, but this is not limitative. The        work vehicle includes also a riding type paddy field        transplanter, a combine-harvester, a riding type mower and the        like.

The invention claimed is:
 1. A transmission switching arrangement for awork vehicle comprising: transmission switching means having a pluralityof hydraulic clutches for switching over a transmission state of a powertransmission line including speed changing means; controlling means forcontrolling operations of the transmission switching means and the speedchanging means; commanding means for commanding, to the controllingmeans, switchover of the transmission state by the transmissionswitching means: and detecting means for detecting a rotational speed ofthe power transmission line; wherein, upon receipt of a command fromsaid commanding means for effecting the switchover of the transmissionstate by the transmission switching means, said controlling meanscompares an output from said detecting means with a set speed fortransmission state switchover which is set in advance; and when theoutput from said detecting means is greater than said set speed, saidcontrolling means causes reduction in the rotational speed of the powertransmission line by a speed reducing operation of the speed changingmeans and then effects the transmission state switchover by controllingpressure of the plurality of hydraulic clutches.
 2. A transmissionswitching arrangement for a work vehicle according to claim 1, whereinsaid speed changing means comprises a mechanical stepless speed changedevice.
 3. A transmission switching arrangement for a work vehicleaccording to claim 1, wherein said controlling means is configured toreduce the rotational speed of the power transmission line to the setspeed for the transmission state switchover by the speed reducingoperation of the speed changing means; and then to effect thetransmission state switchover by controlling pressure of the pluralityof hydraulic clutches.
 4. A transmission switching arrangement for awork vehicle according to claim 2, wherein said controlling means isconfigured to reduce the rotational speed of the power transmission lineto the set speed for the transmission state switchover by the speedreducing operation of the speed changing means; and then to effect thetransmission state switchover by controlling pressure of the pluralityof hydraulic clutches.
 5. A transmission switching arrangement for awork vehicle according to claim 1, wherein said controlling means isconfigured to reduce the rotational speed of the power transmission lineby effecting the speed reducing operation of the speed changing meansfor a set period; and then to effect the transmission state switchoverby controlling pressure of the plurality of hydraulic clutches.
 6. Atransmission switching arrangement for a work vehicle according to claim2, wherein said controlling means is configured to reduce the rotationalspeed of the power transmission line by effecting the speed reducingoperation of the speed changing means for a set period; and then toeffect the transmission state switchover by controlling pressure of theplurality of hydraulic clutches.
 7. A transmission switching arrangementfor a work vehicle according to claim 1, wherein said controlling meansis configured to start the transmission state switchover by controllingpressure of the plurality of hydraulic clutches; and then to cause, byan acceleration operation of the speed changing means, to increase therotational speed of the power transmission line to a speed prior to thespeed reducing operation by the speed changing means.
 8. A transmissionswitching arrangement for a work vehicle according to claim 2, whereinsaid controlling means is configured to start the transmission stateswitchover by controlling pressure of the plurality of hydraulicclutches; and then to cause, by an acceleration operation of the speedchanging means, to increase the rotational speed of the powertransmission line to a speed prior to the speed reducing operation bythe speed changing means.
 9. A transmission switching arrangement for awork vehicle according to claim 3, wherein said controlling means isconfigured to start the transmission state switchover by controllingpressure of the plurality of hydraulic clutches; and then to cause, byan acceleration operation of the speed changing means, to increase therotational speed of the power transmission line to a speed prior to thespeed reducing operation by the speed changing means.
 10. A transmissionswitching arrangement for a work vehicle according to claim 4, whereinsaid controlling means is configured to start the transmission stateswitchover by controlling pressure of the plurality of hydraulicclutches; and then to cause, by an acceleration operation of the speedchanging means, to increase the rotational speed of the powertransmission line to a speed prior to the speed reducing operation bythe speed changing means.
 11. A transmission switching arrangement for awork vehicle according to claim 5, wherein said controlling means isconfigured to start the transmission state switchover by controllingpressure of the plurality of hydraulic clutches; and then to cause, byan acceleration operation of the speed changing means, to increase therotational speed of the power transmission line to a speed prior to thespeed reducing operation by the speed changing means.
 12. A transmissionswitching arrangement for a work vehicle according to claim 6, whereinsaid controlling means is configured to start the transmission stateswitchover by controlling pressure of the plurality of hydraulicclutches; and then to cause, by an acceleration operation of the speedchanging means, to increase the rotational speed of the powertransmission line to a speed prior to the speed reducing operation bythe speed changing means.